The present invention relates to an image reading apparatus with a diffraction grating, and particularly, to an image reading apparatus of a line reading type which comprises a heat radiation type light source, a diffraction grating for color separation, and a line sensor having three reading lines.
In the conventional image reading apparatus with a diffraction grating of a line reading type, when color images in a line on document 1 are optically guided, through lens 2, slit 3 and lens 4, into diffraction grating 5 for color separation, in such an arrangement as shown in FIG. 4, for example, the images are color-separated by the diffraction grating 5 into R(red), G(green) and B(blue) which can be read by the line sensor 6 having three reading lines. The slit 3 is used for restricting the reading width in the scanning direction and thereby for preventing the so-called crosstalk which means that image information from other scanning lines enter the currently scanning line as a noise.
As the diffraction grating 5 which is a color separation element of the above-mentioned apparatus, there is known the diffraction grating element disclosed for example, in Japanese Patent Examined Publication No. 45210/1986.
This diffraction grating element is constructed so that, as shown in FIG. 2, the thickness of a transparent plate such as a glass plate or the like forms steps repeatedly changed along one direction of the plate surface. The difference of thickness of the steps is formed so that the difference in the length of optical paths which is calculated by the multiplication of a selected wavelength by an integer, is produced. For example, the plate thickness is changed in three steps so that color component images of R(red), G(green), and B(blue) can be formed and a zero order green image can be formed on the central optical axis.
As a light source of a conventional color image reading apparatus, the so-called heat radiation type light source is most widely used, in which a filament of a halogen lamp or the like is heated by an electric current running through the filament and light is emitted by the heat radiation.
The heat radiation type light source has advantages in that it is inexpensive, easily available, and it requires no special circuit for lighting.
However, it has disadvantages in that it has the maximum value in the infrared region of the wavelength of about 1 .mu.m, and has the minimum light intensity in the blue region in its spectral distribution characteristics. Further, in the light separation by the diffraction grating element shown in FIG. 2, only a portion of a spread of .+-.1 order diffracted light can be received in comparison with the light receiving of the zero order diffracted light, because a light receiving width of the line sensor 6 is limited. Accordingly, it has the characteristics in which the quantity of the received light becomes small. Consequently, when the diffraction grating element wherein the light in a blue region is .+-.1 order diffracted light, and a heat radiation type light source are combined, a photoelectric output in the blue region is extremely smaller than those in green and red regions. As a result, there is a problem that a well-balanced image signal can not be obtained.
An object of the present invention is to solve the above-mentioned problems, to improve the insufficient sensitivity in the blue region, and to provide an image reading apparatus in which the well-balanced image signal can be obtained.